The human heart periodically contracts to force blood through the arteries. As a result of this pumping action, pressure pulses or oscillations exist in these arteries and cause them to cyclically change volume. The minimum pressure during each cycle is known as the diastolic pressure and the maximum pressure during each cycle is known as the systolic pressure. A further pressure value, known as the “mean arterial pressure” (MAP) represents a time-weighted average of the measured blood pressure over each cycle.
While many techniques are available for the determination of the diastolic, systolic, and mean atrial pressures of a patient, one such method typically used in non-invasive blood pressure monitoring is referred to as the oscillometric technique. This method of measuring blood pressure involves applying an inflatable cuff around an extremity of a patient's body, such as the patient's upper arm. The cuff is then inflated to a pressure above the patient's systolic pressure and then incrementally reduced in a series of small steps. A pressure sensor pneumatically connected to the cuff measures the cuff pressure throughout the deflation process. The sensitivity of the sensor is such that it is capable of measuring the pressure fluctuations occurring within the cuff due to blood flowing through the patient's arteries. With each beat, blood flow causes small changes in the artery volume which are transferred to the inflated cuff, further causing slight pressure variations within the cuff which are then detected by the pressure sensor. The pressure sensor produces an electrical signal representing the cuff pressure level combined with a series of small periodic pressure variations associated with the beats of a patient's heart for each pressure step during the deflation process. It has been found that these variations, called “complexes” or “oscillations,” have a peak-to-peak amplitude which is minimal for applied cuff pressures above the systolic pressure. As the cuff pressure is decreased, the oscillation size begins to monotonically grow and eventually reaches a maximum amplitude. After the oscillation size reaches the maximum amplitude, the oscillation size decreases monotonically as the cuff pressure continues to decrease. Oscillametric data such as this is often described as having a “bell curve” appearance. Indeed, a best-fit curve, or envelope, may be calculated representing the measured oscillometric pulses. Physiologically, the cuff pressure at the maximum oscillation amplitude value approximates the MAP. In addition, complex amplitudes at cuff pressures equivalent to the systolic and diastolic pressures have a fixed relationship to this maximum oscillation amplitude value. Thus, the oscillometric method is based upon measurements of detected oscillation amplitudes at various cuff pressures.
Blood pressure measuring devices operating according to the oscillometric method detect the amplitude of the pressure oscillations at various applied cuff pressure levels. The amplitudes of these oscillations, as well as the applied cuff pressure, are stored together as the device automatically changes the cuff pressures through a predetermined pressure pattern. These oscillation amplitudes define an oscillometric “envelope” and are evaluated to find the maximum value and its related cuff pressure, which is approximately equal to MAP. The cuff pressure below the MAP value which produces an oscillation amplitude having a certain fixed relationship to the maximum value is designated as the diastolic pressure, and, likewise, the cuff pressures above the MAP value which results in complexes having an amplitude with a certain fixed relationship to that maximum value is designated as the systolic pressure. The relationships of oscillation amplitude at systolic and diastolic pressures, respectively, to the maximum value at MAP are empirically derived ratios depending on the preferences of those of ordinary skill in the art. Generally, these ratios are designated in the range of 40%-80% of the amplitude at MAP.
One way to determine oscillation magnitudes is to computationally fit a curve to the recorded oscillation amplitudes and corresponding cuff pressure levels. The fitted curve may then be used to compute an approximation of the MAP, systolic, and diastolic data points. An estimate of MAP is taken as the cuff pressure level with the maximum oscillation. One possible estimate of MAP may therefore be determined by finding the point on the fitted curve where the first derivative equals zero. From this maximum oscillation value data point, the amplitudes of the oscillations at the systolic and diastolic pressures may be computed by taking a percentage of the oscillation amplitude at MAP. In this manner, the systolic data point and the diastolic data point along the fitted curve may each be computed and therefore their respective pressures may also be estimated. This curve fitting technique has the advantage of filtering or smoothing the raw oscillometric data. However, in some circumstances it is been found that additional filtering techniques used to build and process the oscillometric envelope could improve the accuracy of the determination of the blood pressure values.
The reliability and repeatability of blood pressure computations hinges on the ability to accurately determine the oscillation amplitude. However, the determination of the oscillation amplitudes is susceptible to artifact contamination. As the oscillometric method is dependent upon detecting tiny fluctuations in measured cuff pressure, outside forces affecting this cuff pressure may produce artifacts that in some cases may completely mask or otherwise render the oscillometric data useless. One such source of artifacts is from voluntary or involuntary motion by the patient. Involuntary movements such as the patient shivering may produce high frequency artifacts in the oscillometric data. Voluntary motion artifacts, such as those caused by the patient moving his or her arm, hand, or torso, may produce low frequency artifacts.
Presently available systems may be able to determine whether or not collected oscillometric data has been corrupted with artifact; however, current filtering techniques are ineffective at removing artifacts that have similar frequency content as the desired oscillometric data. Alternatively, non-invasive blood pressure systems may simply reject oscillometric data that has been designated as being corrupted by artifacts. In these instances, more oscillometric data must be collected at each pressure step until reasonably artifact free oscillometric data may be acquired. This may greatly lengthen the time for determination of a patient's blood pressure and submit the patient to increased discomfort that is associated with the inflatable cuff restricting blood flow to the associated extremity.